The present invention relates to a motor vehicle gearbox, comprising at least two concentrically arranged alternately driveable first and second input shafts, first and second layshafts, primary gears carried by the first input shaft and the first layshaft, said primary gears engaging each other for transmitting torque from the first input shaft to the first layshaft, primary gears carried by the second input shaft and the second layshaft, said primary gears engaging each other for transmitting torque from the second input shaft to the second layshaft, an output shaft, gears carried by the layshafts and the output shaft, said gears engaged in each other in pairs for transmitting torque from either layshaft to the output shaft and of which at least one gear in each pair is a releasable idle gear, and synchronizing means comprising firstly a pair of synchronizing gears of different diameters rotatably mounted on one of the layshafts, said gears being in driving engagement with gear rings on one input shaft, and secondly friction means, by means of which a driving connection between one synchronizing gear and the shaft can be established to adapt the rotational speed of the layshaft which is driveably coupled to the momentarily undriven input shaft, to a rotational speed determined by the selected gear speed, at least the synchronizing gear of smaller diameter being coordinated with engaging means which permits transmission of higher torque to its layshaft than what is required for said rotational speed adaption.
A gearbox of the above stated type is previously known from SE-A-507 366. It is based on a so-called xe2x80x9cpower-shiftxe2x80x9d gearbox with a two-position central synchronization, whereby six synchronizing functions are obtained in a seven-speed gearbox with two synchronizing gears and a synchronizing clutch on one of the layshafts. It is further developed in that one of the synchronizing gears is made so that it can also be used for torque transmission to provide an additional gear speed, namely an overdrive. Practically, this can be achieved by widening one of the normally quite narrow synchronization gears and providing it with engaging means which provides a shape engagement instead of a friction engagement.
The purpose of the present invention is, in a gearbox of the type described by way of introduction, to increase the number of gear ratios with the smallest possible economic investment and design work.
This is achieved according to the invention by virtue of the fact that that the primary gear of the second layshaft is coordinated with engaging means, whereby the primary gear is releasable from and lockable to its shaft, and that the primary gear is releasable from its shaft at the same time as the synchronizing gear of smaller diameter is lockable by means of its engagement means to its shaft to permit torque transmission from the second input shaft via said synchronization gear and inter-engaging gears on the first and second layshafts to the output shaft.
Merely by arranging the primary gear of the second layshaft to be releasable, which is known per se in other contexts, and coordinating this release with the engagement of the other synchronization gear, i.e. the overdrive gear, a second reverse gear is obtained with a lower gear ratio than the existing reverse gear. To do such design work, only one additional engaging means is required to provide the gearbox the gearbox shown in SE-A-507 366 with an additional gear speed.